Captive piston stud driver



May 4, 1965 R. T. CATLIN ETAL CAPTIVE PISTON STUD DRIVER 3 Sheets-Sheet 1 Filed March 29, 1956 INVENTORS ROBERT T. CATLIN Y JOHN CLARKE S)NEX May 4, 1965 R. r. CATLIN ETAL CAPTIVE PISTON STUD DRIVER s Sheets-Sheet 2 Filed March 29, 1956 m @Pi IN V EN TORS R. T. CATLIN ETAL CAPTIVE PISTON STUD DRIVER May 4, 1965 3 Sheets-Sheet 5 Filed. March 29, 1956 ofiwE wI mm mm mm EN NM m mm g \\\\\\\\\v v1 4 vwvvw I EH A r MN United States Patent M 3,181,760 CAPTIVE PITON STUD DRIVER Robert T. Catlin, Trumbull, Conn., and John Clarke diuex, llion, N.Y., assignors to Remington Arms Company, Inc, Bridgeport, Conrn, a corporation of Delaware Filed Mar. 29, 1956, Ser. No. 574,677 12 Claims. (Cl. 227-) necessary to take precautions against the hazard of ricocheting or fishhooking studs which occasionally result from impact with a fragment of hard aggregate near the surface of a concrete workpiece. There are on record a number of instances of fatal and serious injuries as a result of failure to observe these precautions and for this reason there are some jobs and some areas in which it has not been considered feasible to use explosively driven fasteners.

It is the object of our invention to provide a type of explosively powered fastening device which retains positive control of the fastener until the driving operation has been completed and which may even be discharged free from any contact with a penetration resistant workpiece without permitting the fastener to be projected as a ballistic missile.

It is a further object to provide such an explosively powered fastening device which may be used with ease and convenience.

We believe that the best means of achieving these objects is to provide an explosively driven piston to which the fastening device may be firmly secured and to provide on the driving tool means for retaining both the piston and the fastener in the event the workpiece is not present or offers so little resistance to penetration by the stud as to endanger anyone on the opposite side of that workpiece. By these same means we can avoid endangering the tool operator or others as a result of a ricochet or fishhooking fastener.

We prefer to arrange the means for retaining the piston so that under normal stud driving conditions the stud will be stopped by the penetration resistance of the workpiece and the retainer will not have to absorb any impact. Cnly in the abnormal or dangeroussituation is it necessary for the retainer to absorb the shock and thus the means for preventing the escape of the piston and fastener has a long service life.

The exact nature of the invention as well as other objects and advantages thereof will become more clearly apparent from consideration of the following specification referring to the attached drawings in which:

FIG. 1 is a longitudinalsectional view through a portion of a stud driver embodying our invention. The tool is loaded ready to be placed against a work surface for a stud driving operation.

PEG. 2 is a similar .view showing the'tool still in position against a work surface after a stud has been driven thereinto.

FIG. 3 is a similar View showing the tool as it has been withdrawn from the work surface.

FIG. 4 is a similar view showing the result of firing the tool with no workpiece in place.

FIG. 5 is a similar view showing the mechanism as- Patented May 4, 1965 sembled for firing with the fastening device inadvertently omitted.

FIG. 6 is a cross-sectional view on the line 6-6 of .FIG. 7 is a cross-sectional view on the line 77 of FIG. 1.

PEG. 8 is-an isometric assembly View of the driving piston and fastening device.

FIG. 9 is a side elevational view partially in section showing an alternative form of variable energy driving piston.

FIGS. 10 and 11 are views similar to FIG. 9 showing other embodiments of variable energy pistons.

FIG. 12 is a cross-sectional view on the line 1212 of FIG. 11.

FIG. 13 is a fragmentary sectional view corresponding to the muzzle end of FIG. 1, illustrating a modified form of buffer disk.

FIG. 14 is a cross-sectional view taken in a plane corresponding to 66 of FIG. 1, illustrating a modified form of tool.

FIG. 15 is an isometric assembly view of a modified form of driving piston and fastening device adapted for use in the modified tool shown in FIG. 14-.

PEG. 15 is a cross-sectional view taken on the plane corresponding to 77 of FIG. 1, illustrating the modified tool to which'FlGS. 14 and 15 also refer.

Referring to the drawings by characters of reference, it may be seen that we have embodied our invention in a form suitable for application to a fastener driving tool such as that illustrated in the copending application of Merle H. Walker, Newton M. Reed, and Robert T. Catlin, Serial Number 517,192, filed June 22, 1955, and entitled Powder Actuated Tool, now Patent No. 3,054,110, dated September 18, 1962. Although the illustrations refer specifically to that tool it should be noted that our invention may be applied to any fastener driving tool capable of supporting the barrel 1 and chamber bushing 2 in properrelation to each other and safely confining and 3 firing a cartridge in the chamber bushing. For simplicity, we have here illustrated only the rudimentary parts of such a fastener driving tool eliminating such refinements as safety devices, guards, and the like. r

In the construction shown, the barrel 1 is threadably attached to a frame plate 3, and a chamber plate 4 is provided to support the chamber bushing. As outlined in the copending application above referred to, thprincipal function of the chamber plate is 'to hold the chamber bushing during the intermediate stages of the loading cycle. The breech plate 5 fits against the rear face of the chamber plate and supports the firing mechanism, here indicated only by a firing pin 6. Alocking sleeve 7 en circles the three plates referred to and is provided with interrupted lugs 8 engaging lugs on the breech plate 5 and with similar interrupted lugs 9 which can be brought into engagement with lugs 10 on the frame plate 3 to securely lock thebreech mechanism in cartridge confining relationship.

The chamber bushing 2 is in function and interior construction identical to that disclosed in the copending application of Robert T. Catlin et al., Serial No. 517,524, filed .lune 23, 1955, and entitled, ChamberBusln'ng forStud Driver, now Patent No. 2,759,185, serving to support the cartridge and insure-regularity anduniformity in the combustion of the charge therein.

The construction thus far described is that of the significant elements of the .copending application of Walker;

for example, that shown in the copending application of Robert T. Catlin, Serial No. 526,405, filed August 4, 1955,

circular bore.

enemas U and entitled, Cartridge Powered Tool, new Patent No. 2,768,375.

The barrel 1 is formed with a bore of non-circular, for example, square or triangular cross-section, conveniently onehalf inch on a side, and the driving piston Ill used therein is provided with a head 12 of matching noncircular form having a substantially gas-tight sliding fit in the bore of the head. The forward portion of the piston 11 may conveniently be of circular cross-section having tangential engagement with walls of the non- To insure that the piston will be frictionally retained in any desired position in the bore of the barrel a spring detent 13, best seen in FIG. 8, is provided. This detent is formed of spring wire bent into the form of a four-pointed star, the points of which project through milled slots 14 extending from the outer surface of the non-circular piston head into intersection with the axial bore 15 in the piston. The axial bore 15 extends completely through the piston and is threaded at the front end to receive the threaded end 16 of a stud l7. To provide for a substantially flush head after driving, the stud illustrated may be necked down as at lid to permit breaking off the threaded end after the stud has been driven.

Obviously, any of the common threaded studs may be used without alteration but for those ordinarily having plain heads it is necessary to provide a threaded portion as a holding means.

The muzzle end of the barrel is threaded to receive a muzzle cap 19 and in the area immediately forward of the muzzle of the barrel the bore of the muzzle cap is of circular cross-section slightly larger in diameter than the greatest dimension of the non-circular bore. The circular bore in the muzzle cap receives a buffer disk 2! which has in it a non-circular bore only slightly, if any, larger than the non-circular bore of the barrel, and the forward end of the muzzle cap is provided with a flange 21 having a non-circular hole therein aligned with the bore in the bufier disk to back up the disk. In assembly, the noncircular bore in the muzzle cap is lined up out of registry with the non-circular bore of the barrel so that a major dimension of the bore of the barrel is aligned with a minor dimension of the bore in the disk. For example, the projections of the corners of the square bore of the barrel lie midway between the corners of the square bore of the buffer disk. The muzzle cap may then be locked in position by the locknut 22.

In operation, the stud driver may be loaded as in FIG. 1 with the cartridge in the chamber bushing and a stud screwed into the piston. When the muzzle is placed adjacent a work surface W and the cartridge fired, the piston will drive the stud into the work surface as shown in FIG. 2 and the non-circular head of the piston will come to rest either still in the bore of the barrel or in the circular bore of the muzzle cap between the muzzle of the barrel per se and the bufier disk.

To raise the tool from the work, it is only necessary to rotate the barrel a sufficient amount turn in the case of the square bore) to align the bore in the buffer disk and backing flange with the non-circular head of the driving piston, and the barrel of the tool may be lifted free of the driven stud as shown in FIG. 3. The driving piston may then be manually unscrewed from the driven .stud and fitted with a new stud for a subsequent driving operation.

Should the tool be fired when not in contact with the work or fired against an easily penetrated surface such as a plaster wall assumed to be solid concrete, the noncircular end of the driving piston will emerge from the muzzle of the barrel and will impinge upon the buffer disk, the major dimensioned portions of the piston striking in substantially the middle of the minor dimensioned portions of the buffer disk. In this situation, illustrated in FIG. 4, the piston penetrates for a substantial distance into the buffer disk which is conveniently made of a yieldable metallic material.

The tool is, in the event described in the preceding paragraph, temporarily disabled but this is not serious for the buffer disk is only utilized as the safety means for stopping a shot which might otherwise have caused physical injury. in any event, the piston may be readily driven or forced out of the buffer disk and with suitable tools the deformed buffer disk may be driven out of the muzzle cap and a replacement buffer disk applied thereto. Alternatively, a supply of muzzle caps with bulfcr disks in place may be kept on hand by stud driver users and distributors and replacements made on an exchange basis which permits replacement of buffer disks in a shop where facilities are generally better than in the field.

Conveniently, the buifcr disk may be made of solid aluminum and will have sufficient strength to consistently stop the assembly of piston and stud when need be and will yield with sufiicient ease to prevent damage due to i the application of excessive force to the contacting portion of the piston.

It has been discovered that there is a marked difference in the static and dynamic tests on a disk of a solid aluminum alloy (24S-T). In a static test of a disk of this type, the loading on the corners of the piston increases very rapidly, approaching the force required to shear the corners of the piston at just over .100 penetration, yet when the piston is fired against the disk, using the heaviest charge, it will penerate from .300 to .400", without damage to the corners of the piston or any part of the tool other than the disk itself. By the condition of the butler disk after a test shot, it appears that the velocity of impact of the piston builds up enough frictional heat to cause a plastic flow condition in the aluminum, which allows the piston to penetrate deeper, thus dissipating more energy. A series of test shots were run using solid aluminum buffer disk of llST alloy in some cases and of 248-7? alloy in other cases. These shots were fired in the captive piston stud driver with the muzzle of the barrel positioned over a hold in a firing pit so that the full energy of the piston was imparted directly to the buffer disk. Penetration into the disk was found to have a direct relationship to the power of the cartridge, i.e., maximum .power load gave maximum penetration; however, there appeared to be no exact correlation between the amount of penetration and the weight of the piston-stud combination for a given powder charge.

We have also found that a composite buffer disk comprising a comparatively hard shearable material such as steel and a relatively soft thicker disk of such a material as aluminum is quite useful in many situations. When in use as a buffer system, the piston contacts the hard disk first causing the loading to increase rapidly until the hard disk fails in shear. As penetration continues, the piston carries the sheared fragments of the hard disk forward, deforming the softer disk ahead of it and maintaining a relatively high loading. The nature of this loading cycle is such that the maximum amount of energy will be absorbed for the minimum penetration of the piston.

During normal firing use, the harder disk serves a sec ondary function in that its more deformation-resistant surface offers more resistance to abrasion or indentation by the piston as the tool is lifted from the work surface.

It can be seen that significant variations in the loading cycle and energy absorption can be achieved by varying the thickness and/ or hardness of either disk and all without sacrificing the secondary function of the hard disk.

In the event that an operator negligently fails to apply a stud to the piston before reloading and firing, as illustrated in FIG. 5, it will be noted that the piston is bored axially for its complete length. In this event, the gases generated by the firing of the cartridge will escape harmlessly through the bore of the piston and the piston, if it moves at all, will only be weakly projected down the barrel to stop without contacting the buffer disk and without damage to the piston as the front end thereof impinges on the work surface. Even in the situation where such a shot was fired with a non-resistant work surface opposed to the muzzle, the piston will not be projected against the buffer disk with sufiicient velocity to do damage to the piston, and will only lightly indent the buffer disk.

The modified forms of driving pistons shown in FIGS. 9 through 11 are all intended to provide for variable driving power and at the same time permit the utilization of a single comparatively high-powered cartridge. These modifications provide means to effect, on the pistons, variations in the apparent free volume in which the cartridge is fired, or variations in the degree of confinement of the gas, or both effects may be realized. In each case it is possible to provide for a range of driving power comparable to that resulting from the selective use of cartridges of different power.

The piston 23 shown in FIG. 9 has an axial bore 24 which is threaded to receive a hollow screw 25 formed internally to receive the conventional hexagonal Allen wrench as a means of adjustment. The hollow screw 25 functions as a threadably adjustable sleeve valve to cut oif varying numbers from zero to all of a plurality of gas vents 26 which are positionedto bleed off gas into the square corners of the barrel. Preferably, these holes are spaced at 90 intervals and their location and diameter is so related to the pitch of the thread that another vent hole is cut off by each revolution of the valve screw. Obviously, when all vents are open there will be minimum power delivered to the stud and when all vents are closed there will be maximum power available for stud driving. In this modification a detent plug 27 of a material such as the synthetic tetrailuorethylene resin may be provided for frictional engagement with a corner of the square bore of the barrel to perform the same function as the wire detent referred to previously.

In the form shown in FIG. 10, the driving piston 25 is similar to that shown in FIG. 9, with the major change being the longitudinal extension of the sleeve valve 2% to extend rearwardly behind the square end of the piston, being there provided with a knurled fiange St) to facilitate manual adjustment. The rearward movement of this sleeve valve not only has the effect of opening up more of the vent holes 31 but has the further significant effect of increasing the free volume in the system at the time the cartridge is fired. Thus, both the greater expansion volume and venting are susceptible of exact control and available as means of controlling power from a single standard cartridge.

In the form shown in FIG. 11, the driving piston 32 is fitted with a pair of telescoping hollow screws 33 and 34 which provide a large degree of control over initial free volume in the gas expansion system.

FIGS. 14, 15, and t6 illustrate another form of noncircular bore and piston to demonstrate that our invention is not limited to the square form illustrated in the other figures. In this case, the bore of the barrel 1A and the head 12A of the piston have the form of a flattened ellipse, the minor axis of the ellipse being equal to the diameter of the forward extension 11A of the piston to engage and guide the piston through its length as it travels through the barrel. The buffer disk ZtlA and muzzle cap 19A have a bore of the same form as the barrel but are indexedat 90 to the barrel so that the major axis of the bore of the barrel is coincident with the minor axis of the bore of the buffer disk and muzzle cap. In the space between the buffer disk and the muzzle of the barrel per se, a circular chamber is formed in the muzzle cap having a slightly larger diameter than the major axis of the elliptical bore.

The wire detent 13A provided in this form of piston projects through milled slots 14A which communicate between the exterior surface of the piston head 12A and the longitudinal bore 15A therein and has the same function as the detents provided in the other modifications.

Although we have not here attempted to show a wide variety of modifications, it'should be apparent that there 6 are other embodiments which within the meaning of the appended claims can be considered to be equivalents. For an exact definition of the limits upon our invention, reference may be made to the claims herein.

We claim:

1. A tool for driving fastening devices with the power derived from an explosivecartridge comprising in combination a barrel having a bore of non-circular crosssection, a chamber in said barrel in which an explosive cartridge can be fired, a driving piston slidable in said bore and fitted with a non-circular head having substantially gas tight engagement with the bore, manually releasable means resistant to any force in either tension or compression resulting from the inertia of the fastener, said releasable means acting to secure together said driving piston and a fastener in spite of inertia and other forces resulting from the explosive driving of said piston or from interception and stopping of said driving piston, and a muzzle cap secured to the end of said barrel, said muzzle cap having formed therein an aperture through which the fastening device may be driven and with means to intercept the non-circular head of the driving piston and prevent the explosive expulsion of said driving piston and the fastener secured thereto as free ballistic missiles, said muzzle cap having therein a non-circular aperture of the same form as the non-circular bore, the muzzle cap being so oriented with respect to the bore of the barrel that the projections of the major axis of the bore of the barrel oppose the minor axis of the non-circular aperture in the muzzle cap so that the head of the piston may not pass through the cap without reorientation after emerging from the muzzle of the barrel.

2. A tool as defined in claim 1, the apertured portion of the muzzle cap being spaced from the muzzle of the barrel by a portion of the cap having an inside diameter greater than the major axis dimension of the non-circular bore and a length greater than the length of the noncircular head on the driving piston, to permit manual reorientation of the driving piston and removal thereof through the muzzle cap aperture.

3. A tool as defined in claim 2, said muzzle cap being provided with a shock absorbing buffer disk formed of deformable metal with a non-circular aperture extending therethrough in registry with the non-circular aperture in the muzzle cap, said bulfer disk being adapted to intercept and absorb the impact ofthe head of the piston if driven thereagainst.

4. A tool as defined in claim 3, said non-circular members being of polygonal form and so oriented that the projection of the cornersof the'polygonal bore of the barrel intersect midway on the sides of the bore of the polygonal aperture in the muzzle cap.

5. A tool as defined in claim 3, said buffer disk bein formed of a solid deformable alloy of aluminum.

6. A tool as defined in claim 3, said buffer disk being a composite formed of a solidmetal washer of relatively hard shearable metal facing the driving piston and backed up by a relatively thick solid disk of a relatively soft deformable metal.

7. A tool as defined in claim 6, said shearable metal washer being of steel and said thick diskof aluminum.

8. A tool for driving fastening devices with the power derived from an explosive cartridge comprising in combination a barrel having a bore of non-circular crosssection, a chamber in said barrelin which an explosive cartridge can be fired, a driving piston slidable in said bore and fitted with a non-circular head having substantially gas tight engagement with the bore, manually releasable means resistant to any force in either tension or compression resulting from the inertia of the fastener, said releasable means acting to secure together said driving piston and a fastener in spite of inertia and other forces resulting from the explosive driving of said piston or from interception and stopping of said driving piston,

and a muzzle cap secured to the end of said barrel, said.

muzzle cap having formed therein an aperture through which the fastening device may be driven and with means 'to intercept the non-circular head of the driving piston and prevent the explosive expulsion of said driving piston and the fastener secured thereto as free ballistic missiles, said non-circular members being of polygonal form and so oriented that the projection of the corners of the polygonal bore of the barrel intersect the sides of the aperture in the muzzle cap.

9. A tool for driving fastening devices with the power derived from an explosive cartridge comprising in combination a barrel having a bore of non-circular crosssection, a chamber in said barrel in which an explosive cartridge can be fired, a driving piston slidable in said bore and fitted with a non-circular head having substantially gas tight engagement with the bore, manually releasable means resistant to any force in either tension or compression resulting from the inertia of the fastener, said releasable means acting to secure together said driving piston and a fastener in spite of inertia and other forces resulting from the explosive driving of said piston or from interception and stopping of said driving piston, and a muzzle cap secured to the end of said barrel, said muzzle cap having formed therein an aperture through which the fastening device may be driven and with means to intercept the non-circular head of the driving piston and prevent the explosive expulsion of said driving piston and the fastener secured thereto as free ballistic missiles, said driving piston being provided with a longitudinal bore extending completely through the piston, which bore is closed olf only when the piston has attached to it the fastener to be driven, said driving piston being provided with a plurality of gas vents extending from said longitudinal bore to the space in said barrel forward of the non-circular head of said driving piston, and a manually adjustable valve member in said longitudinal bore to permit the selective opening of said vents to bypass gas through said piston and thereby reduce the force applied to the fastener to be driven.

10. A tool as defined in claim 9, said manually adjustable valve member being threadably mounted in said longitudinal bore and provided with a head extending beyond the rear face of the driving piston head, said gas vents being so arranged that threadable adjustment of said valve member to cause it to extend further beyond the rear face of the driving piston head and thereby to increase the effective combustion chamber volume simultaneously opens more of the vents.

11. A tool for driving fastening devices with the power derived from an explosive cartridge comprising in combination a barrel having a bore of non-circular crosssection, a chamber in said barrel in which an explosive cartridge can be fired, a driving piston slidable in said bore and fitted with a non-circular head having substantially gas tight engagement with the bore, manually releasable means resistant to any force in either tension or compression resulting from the inertia of the fastener, said releasable means acting to secure together said driving piston and a fastener in spite of inertia and other forces resulting from the explosive driving of said piston or from interception and stopping of said driving piston, and a muzzle cap secured to the end of said barrel, said muzzle cap having formed therein an aperture through which the fastening device may be driven and with means to intercept the non-circular head of the driving piston and prevent the explosive expulsion of said driving piston and the fastener secured thereto as free ballistic missiles,

said driving piston being provided with a longitudinal bore extending completely through the piston, which bore is closed off only when the piston has attached to it the fastener to be driven, said driving piston being provided with means threadably mounted in the longitudinal bore in the piston and manually adjustable to vary the extent of the protrusion of said means beyond the rear face of the riving piston head, said means serving as a spacer to vary the position of the driving piston with relation to the explosive cartridge and thereby to vary the effective combustion chamber volume.

12. An explosive operated tool for projecting fastener devices at high velocities into a work surface, said tool comprising a barrel, a chamber in said barrel in which an explosive cartridge may be fired, a driving piston in said barrel and directly acted upon by explosion gases released by the firing of a cartridge, threaded means interconnecting said driving piston and a fastener device whereby the interconnected driving piston and fastener device may together be directly accelerated by explosive forces, safety means mounted in prolongation of said barrel to intercept said driving piston and limit the movement thereof during and after projection of the fastener device, cooperating means on said driving piston and said safety means to permit manual release of said driving piston from the safety means without requiring disassembly of said safety means from said tool whereby said tool may be removed from the driven interconnected fastener and driving piston and the driving piston may be unscrewed from said fastener, and energy absorbing permanently deformable butler means supported in said safety means and acting between said driving piston and the safety means to cushion the shock of intercepting said driving piston in the event that the interconnected fastener device and driving piston are not stopped by engagement of said fastener device with a work surface.

References Cited by the Examiner UNITED STATES PATENTS 346,720 8/86 Carlson 3092 1,275,028 8/18 Holter 1--44.5 X 1,365,870 1/21 Temple 1-44.5 1,984,117 12/34 Davis 144.5 X 2,008,362 7/35 Littelhale 144.5 X 2,064,503 12/36 Temple 144.5 X 2,221,157 11/40 Temple 144.5 X 2,224,475 12/40 Evans 30910 2,499,227 2/50 Miles 1-445 X 2,666,252 1/54 Temple 144.5 X 2,723,532 11/55 Smith 61-77 2,724,116 11/55 Termet 144.5 2,767,398 10/56 Pfaff et a1 1-44.5 2,775,762 1/ 57 Hilti 144.5

FOREIGN PATENTS 178,075 4/54 Austria.

(Corresponding Great Britain 744,822, February 1956) 62,135 12/54 France.

(2nd addition 1,034,612) 1,055,269 10/53 France.

(Corresponding Great Britain 729,824, May 1955) 1,117,509 2/56 France.

(Corresponding Great Britain 755,764, August 1956) GRANVELLE Y. CUSTER, in, Primary Examiner.

LEWIS I. LENNY, EDWARD V. BENHAM, Examiners. 

1. A TOOL FOR DRIVING FASTENING DEVICES WITH THE POWER DERIVED FROM AN EXPLOSIVE CARTRIDGE COMPRISING IN COMBINATION A BARREL HAVING A BORE OF NON-CIRCULAR CROSSSECTION, A CHAMBER IN SAID BARREL IN WHICH AN EXPLOSIVE CARTRIDGE CAN BE FIRED, A DRIVING PISTON SLIDABLE IN SAID BORE AND FITTED WITH A NON-CIRCULAR HEAD HAVING SUBSTANTIALLY GAS TIGHT ENGAGEMENT WITH THE BORE, MANUALLY RELEASABLE MEANS RESISTANT TO ANY FORCE IN EITHER TENSION OR COMPRESSION RESULTING FROM THE INERITIA OF THE FASTENER, SAID RELEASABLE MEANS ACTING TO SECURE TOGETHER SAID DRIVING PISTON AND A FASTENER IN SPITE OF INERTIA AND OTHER FORCES RESULTING FROM THE EXPLOSIVE DRIVING OF SAID PISTON OR FROM INTERCEPTION AND STOPPING OF SAID DRIVING PISTON, AND A MUZZLE CAP SECURED TO THE END OF SAID BARREL, SAID MUZZLE CAP HAVING FORMED THEREIN AN APERTURE THROUGH WHICH THE FASTENING DEVICE MAY BE DRIVEN AND WITH MEANS TO INTERCEPT THE NON-CIRCULAR HEAD OF THE DRIVING PISTON AND PREVENT THE EXPLOSIVE EXPULSION OF SAID DRIVING PISTON AND THE FASTENER SECURED THERETO AS FREE BALLISTIC MISSILES, SAID MUZZLE CAP HAVING THEREIN A NON-CIRCULAR APERTURE OF THE SAME FORM AS THE NON-CIRCULAR BORE, THE MUZZLE CAP BEING SO ORIENTED WITH RESPECT TO THE BORE OF THE BARREL THAT THE PROJECTIONS OF THE MAJOR AXIS OF THE BORE OF THE BARREL OPPOSE THE MINOR AXIS OF THE NON-CIRCULAR APERTURE IN THE MUZZLE CAP SO THAT THE HEAD OF THE PISTON MAY NOT PASS THROUGH THE CAP WITHOUT REORIENTATION AFTER EMERGING FROM THE MUZZLE OF THE BARREL. 